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Gene Discovery May Lead to New Vaccines

Health: UC Santa Barbara scientists find a 'switch' that controls bacteria's ability to attack its host.


UC Santa Barbara researchers say that they have identified a gene that could make possible inexpensive, powerful vaccines or new antibiotics against the bacteria that cause food poisoning, plague, cholera, dysentery and syphilis.

The discovery comes at a time when a growing number of infectious agents are becoming resistant to humankind's arsenal of antibiotics.

Geneticist Michael J. Mahan and his UC Santa Barbara colleagues report in today's Science that salmonella bacteria carry a gene called dam that serves as an on / off switch for a variety of weapons used by the bacterium to produce disease when it infects humans.

Bacteria lacking the gene do not cause disease, they found, but stimulate a strong immune response, making them ideal ingredients for a vaccine.

Mice immunized with the salmonella mutant were, without exception, able to withstand massive injections of disease-producing bacteria.

An estimated 2 million to 4 million cases of food poisoning alone are produced each year in the United States by such bacteria, and a safe, effective vaccine could reduce that number virtually to zero.

Worldwide, salmonella and other bacteria cause 17 million deaths annually, more than three times the number caused by cancer.

Dam is present in a variety of other dangerous bacteria as well, Mahan says, suggesting that vaccines could also be produced against them.

Human trials could conceivably begin in only a couple of years, he added. Pharmaceutical companies may also be able to develop drugs that block the activity of dam, producing new antibiotics to which the bacteria could not become resistant.

The new discovery "represents a potential breakthrough in our battle against pathogenic bacteria," said Dr. Philip Hanna of Duke University Medical Center.

Researchers have been searching frantically for new ways to combat bacteria, because the most common infectious agents are becoming resistant to all the widely used antibiotics. The antibiotic of last resort in such cases is vancomycin, and scientists around the world have recently identified a handful of sick patients with infections resistant even to it, leaving them without treatment regimens.

"Considering that we are but one antibiotic away from reentering the pre-antibiotic era, the Mahan studies offer fresh insights into combating infectious disease," said Thomas A. Cebula of the Food and Drug Administration.

Mahan and his colleagues began looking for dam based on the observation that a bacterium like salmonella growing in a test tube or in potato salad in a picnic basket is quite different from the same bacterium growing in a mouse or human. In the test tube or the salad, the bacterium is docile, not releasing toxins or activating its other offensive weapons.

But when the same bacterium finds itself in the gut of a mouse or a human, it switches on "virulence" genes that help the microbe penetrate the lining of the gut, travel throughout the body, and use the host's own resources to grow and multiply.

Mahan and his colleagues, including molecular biologists Robert L. Sinsheimer and David A. Low and graduate student Douglas M. Heithoff, identified at least 250 virulence genes in Salmonella typhimurium, the rod-shaped bacterium that colonizes meat and produces food poisoning.

Looking for the on / off switch that activates these genes, they discovered dam, which controls the activity of about 20 virulence genes by chemically modifying key segments of the genes.

They then created two mutants of S. typhimurium in which the dam gene was deleted or disabled by genetic engineering techniques. To their surprise, the mutants produced no disease, but strongly stimulated the host's immune system to kill it--a process that primes the organism to repel future infections by any form of the bacterium.

The team then immunized 25 mice with the mutants, and five weeks later injected the animals with a massive dose of pathogenic S. typhimurium--50,000 times the normal dose required to kill at least half the animals. None were harmed.

"That's pretty compelling," Mahan said.

An additional 10 to 20 mice were immunized with the mutant and then tested with an equally massive dose of the salmonella strain that normally infects chickens and eggs. Again, there was 100% protection, he said.

Dam is also found in a variety of other bacteria, including Vibrio cholerae (cholera), Yersinia pestis (plague), Shigella (dysentery), Haemophilus influenzae (meningitis) and Treponema pallidum (syphilis), as well as the salmonella strains that cause typhoid. Mahan does not yet know if the gene plays the same role in those other bacteria, but he is making the appropriate mutants to find out.

The current salmonella mutant is probably not a good candidate for a human vaccine, Mahan said.

Before testing it in humans, he added, his research team wants to introduce another genetic defect that would prevent the mutant bacteria from causing disease even if it should somehow reacquire the dam gene from another bacterium in an immunized person.

In the meantime, he added, it might be possible to use the existing mutant to vaccinate cattle and chickens. If the animals could not serve as a host for the bacteria, then the chances of humans contracting the bacteria from food would be substantially reduced.

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